1. Field of the Invention
The present invention relates to an optical glass fiber coating composition suitable for use as a primary coating on an optical glass fiber. More particularly this invention relates to an optical glass fiber coating composition having a low viscosity, exhibiting superior storage stability over an extended period of time, and providing a cured material having various excellent characteristics.
2. Description of Related Art
An optical fiber is provided with a resin coating with the object of protecting and reinforcing the bare optical fibers threads. The coating is applied immediately after hot melt spinning of the glass fiber. A structure consisting of a flexible primary coating layer on the surface of the optical fiber and a secondary coating layer outside of this primary coating layer is known as such a resin coating.
The coating material used to form these coatings must have the following characteristics:
(1) be a liquid at room temperature and have high processability; PA0 (2) provide good productivity at a high curing rate; PA0 (3) exhibit very little physical change during temperature changes over a wide range; PA0 (4) have superior heat resistance and superior resistance to hydrolysis; PA0 (5) show superior long term storage stability with little physical changes over time; PA0 (6) show superior resistance to chemicals such as acids and alkalis; PA0 (7) exhibit low moisture and water absorption; PA0 (8) exhibit superior ultraviolet light (UV) resistance; PA0 (9) have excellent oil resistance; PA0 (10) generate a minimal amount of hydrogen gas which affects optical fibers PA0 (11) have superb yellowing resistance against light and heat; and PA0 (12) exhibit only slight shrinkage during curing. PA0 (A) a urethane (meth)acrylate polymer, PA0 (B) a (meth)acrylate compound represented by the following formula (1): ##STR2## PA0 (C) a reaction diluent, and PA0 (D) a polymerization initiator. PA0 40-60 weight % of (A), PA0 15-45 weight % of (B), PA0 and as component (C):
A number of radiation-curable liquid resin compositions have been developed in an attempt to satisfy these requirements.
FR-A-2629218, for instance, describes an optical glass fiber coating composition suitable for use as a primary coating on an optical glass fiber, comprising a urethane acrylate oligomer and 15 to 40 weight % of isobornylacrylate as reactive acrylate diluent. The coating of FR-A-2629218 has a good adhesion to glass, gives suitable protection of the glass fiber and has a high curing speed.
In order to promote the productivity of optical fibers, it is essential that the drawing speed from melted glass fiber materials be elevated. At a high drawing speed, however, the conventional optical glass fiber coating compositions have a problem of generating a large quantity of volatile matters due to polymerization heat or radiation heat from radiation sources such as an ultraviolet lamp or the like.
U.S. Pat. No. 4,992,524 discloses an acrylate urethane oligomer, in combination with a reactive diluent and a photoinitiator. In the list of reactive diluents, at column 6, lines 1-42 2-hydroxy-3-phenoloxypropyl(meth)acrylate is mentioned. However, the disclosure of (U.S. Pat. No. 4,992,524) is absent of any specific preference for this compound, nor is there any teaching which would lead the skilled man to use this compound, in combination with other reactive diluents, in order to make coating compositions that exhibit low weight reduction (of the liquid coating composition).